Does every experiment have a manipulated, controlled, and responding variable

My project is to drop ten drops of phenol red (a pH indicator) to five different liquids and to record what colour each liquid changes.

Now I know that the responding variable is the colour that the liquids turn into, but what about the other two?

The manipulated variable can't be the number of drops because every liquid must have exactly ten drops. I'm really confused.

Thanks.

manipulated - five different liquids...

controlled - type of indicator...


responding variable - colour...